The present application relates to a negative electrode for use in a nonaqueous electrolyte secondary cell, a method of manufacturing the negative electrode, and a nonaqueous electrolyte secondary cell using the negative electrode. More particularly, the present application relates to a negative electrode for a nonaqueous electrolyte secondary cell which electrode is foil-like or sheet-like in shape and suitable for being formed into a thin foil and for enhancing capacity, a method of manufacturing the negative electrode, and a nonaqueous electrolyte secondary cell using the negative electrode.
In recent years, attendant on the tendency toward a still higher capacity of nonaqueous secondary cells represented by lithium ion secondary cells, an increase in the loading weight of an electrode composition per cell by enhancing the volume density (composition density) of the electrode composition has been performed as a technique for increasing the capacity.
For the negative electrodes in the nonaqueous electrolyte secondary cells of the above-mentioned type, carbon materials such as graphite are widely used. The carbon materials show a factor of volume expansion attendant on charging of about 1.2, which value is smaller than those of silicon and tin-based alloy negative electrode materials. Therefore, the carbon materials promise higher-capacity cells on a theoretical basis, if such defects as cutting of the electrode plate or peeling or cracking of an electrolyte layer (negative electrode composition layer) are absent.
Under such circumstances, in connection with nonaqueous electrolyte secondary cells using a carbon material for a negative electrode, various improvements in negative electrode characteristics have been attempted, such as the use of a flexible rubber component, e.g., styrene-butadiene rubber, for the negative electrode.
For instance, a method of manufacturing a negative electrode wherein carboxymethyl cellulose is used as a thickener and a styrene-butadiene rubber which is a rubber polymer is used as a binder has been widely known (see, for example, Japanese Patent No. 3286516, which is hereinafter referred to as Patent Document 1). In this method, a negative electrode composition slurry is prepared as a water-soluble dispersion, and, after coating with the dispersion and drying, the negative electrode is molded.
On the other hand, polyvinylidene fluoride widely used as a binder in fabrication of a negative electrode is not soluble in water. In view of this, a method in which a polyvinylidene fluoride powder is mixed into an aqueous slurry containing carboxymethyl cellulose and styrene-butadiene rubber as main ingredients has also been proposed (see, for example, Japanese Patent Nos. 3615472 and 3621031, which are hereinafter referred to as Patent Documents 2 and 3, respectively).
On the contrary, a method in which a solution prepared by using N-methylpyrrolidone (which is widely used as an organic solvent) and containing polyvinylidene fluoride as a main ingredient is admixed with a styrene-butadiene rubber powder having chain terminals modified by a nitrile group or with a styrene-butadiene rubber powder (which case is not clearly described, though) has also been proposed (see, for example, Japanese Patent Laid-open No. Hei 11-214012, which is hereinafter referred to as Patent Document 4).
Meanwhile, carboxymethyl cellulose having a cellulose skeleton as a main chain is a water-soluble thickener and, hence, it is used as an aqueous slurry forming agent or as a binder at the time of molding a negative electrode composition layer. When the thus molded negative electrode plate is subjected to a heat treatment in a non-oxidizing atmosphere, new linkages including ether linkages based on cellulose ring opening or on side chains are formed. It is known that, as a result of the formation of new linkages, an increasing effect on the lithium ion insertion/extraction efficiencies similar to that of a solid-liquid interface layer and a neutralizing effect on carbon surface hydroxyl groups due to a weak-acidifying effect at the time of heat treatment are obtained. Accordingly, an improving effect on charge-discharge characteristics and a suppressing effect on gas generation during first-time charging are exhibited, as known (see, for example, Japanese Patent No. 3191614, which is hereinafter referred to as Patent Document 5).